Cyanides in the form of free cyanide salts or cyanide complexes are potentially toxic compounds that do not normally occur in domestic wastewater. Such cyanides are frequently found in industrial waste effluents, such as in wastewater from gasification plants. It is essential that the cyanide and formate levels in these wastewater effluents are controlled, but adequate methods are not presently available, especially for ferricyanide complexes that are stable and resist chemical means for their destruction.
Current federal regulations limit the concentration of cyanide discharge and the chemical oxygen demand on waste water. The common cyanide discharge limit is 0.2 mg/L. There is no specific discharge limit for formate however, formate contributes to COD (chemical oxygen demand). The common COD discharge limit is 120 mg/L.
High pressure, high temperature gasification systems are used to partially oxidize hydrocarbonaceous fuels to recover useful by-products or energy. The partial oxidation reaction is carried out under high temperature and high pressure conditions, converting about 98% to 99.9% of the hydrocarbonaceous feedstock to a synthesis gas containing carbon monoxide and hydrogen, also referred to as synthesis gas or syngas. Carbon dioxide and water are also formed in small amounts. Water is further used as quench water to quench the syngas, to scrub particulate matter from the syngas, and to cool and/or convey particulate waste solids, such as ash and/or slag out of the gasifier. In order to conserve water, gasification units reuse most of the quench water. However, a portion of the water is normally continuously removed as a wastewater stream in order to prevent excessive buildup of solid materials and undesired dissolved solids.
The composition of the wastewater discharged from the gasification system is fairly complex. The main contaminants of gasification wastewater include sulfides, cyanides, suspended solids (soot, ash fines, and metals), ammonias, and formates. The Texaco Gasification Wastewater Treating Process, as generally described in U.S. Pat. No. 4,211,646 the contents of which are hereby incorporated by reference, is currently used in several operating plants. This process utilizes ferrous sulfate and caustic addition, clarification, filtration, steam stripping, and biological treatment to remove the above contaminants. Drawbacks to this process include difficulty in removing cyanides to less than detectable levels and careful attention is required to maintain a food source (formate) for the biotreatment process.
Oxidation processes for treating cyanides in wastewater also exist in the prior art. One commercial process for the destruction of free cyanide ion in wastewater oxidizes cyanide ions with hydrogen peroxide in the presence of a soluble catalyst, such as copper or iron, at a pH of between 8.3 and 11. However, this process has proven to be ineffective in destroying iron cyanide complexes, such as the ferricyanide ion or the ferrocyanide ion, which is typically found in industrial wastewater streams. Another commercial process destroys cyanide ions in an aqueous solution by first adjusting the pH of the solution to between 11 and 12, then irradiating the solution with ultraviolet (UV) radiation, then reducing the solution to between 8 and 11, and finally adding hydrogen peroxide to initiate oxidation. The present invention is intended to improve upon these existing methods.